This invention generally relates to techniques for spatial compounding of ultrasound image frames and more specifically for using multiple view angles to achieve such spatial compounding.
Conventional ultrasound imaging systems comprise an array of ultrasonic transducer elements arranged in one or more rows and driven with separate voltages. By selecting the time delay (or phase) and amplitude of the applied voltages, the individual transducer elements in a given row can be controlled to produce ultrasonic waves which combine to form a net ultrasonic wave that travels along a preferred vector direction and is focused at a selected point along the beam. The beamforming parameters of each of the firings may be varied to provide a change in maximum focus or otherwise change the content of the received data for each firing, e.g., by transmitting successive beams along the same scan line with the focal point of each beam being shifted relative to the focal point of the previous beam. In the case of a linear array, a focused beam directed normal to the array is scanned across the object by translating the aperture across the array from one firing to the next.
The same principles apply when the transducer probe is employed to receive the reflected sound in a receive mode. The voltages produced at the receiving transducer elements are summed so that the net signal is indicative of the ultrasound reflected from a single focal point in the object. As with the transmission mode, this focused reception of the ultrasonic energy is achieved by imparting separate time delay (and/or phase shifts) and gains to the signal from each receiving transducer element.
A single scan line (or small localized group of scan lines) is acquired by transmitting focused ultrasound energy at a point in the region of interest, and then receiving the reflected energy over time. The focused transmit energy is referred to as a transmit beam. During the time after transmit, one or more receive beamformers coherently sum the energy received by each channel, with dynamically changing phase rotation or delays, to produce peak sensitivity along the desired scan lines at ranges proportional to the elapsed time. The resulting focused sensitivity pattern is referred to as a receive beam. A scan line""s resolution is a result of the directivity of the associated transmit and receive beam pair.
A B-mode ultrasound image is composed of multiple image scan lines. The brightness of a pixel is based on the intensity of the echo return from the biological tissue being scanned. The outputs of the receive beamformer channels are coherently summed to form a respective pixel intensity value for each sample volume in the object region or volume of interest. These pixel intensity values are log-compressed and scan-converted to form an image frame of pixel data which can be displayed on a monitor.
Multiple scans are performed in succession and multiple image frames are displayed at the acoustic frame rate on the display monitor. In the case where the sonographer is manipulating the transducer probe by hand, any change in the angle (i.e., wobble) of the probe during scanning will cause corresponding changes in the angle at which the transmit beam impinges on the biological tissue being imaged. The result is that some tissue will appear bright during one scan at one angle and dark during another scan at another angle. To compensate for this angle-dependent fluctuation in the intensity of the echo signals reflected by the tissue, it is well known to combine the bright regions from successive image frames to form a compound image. This process is known as spatial compounding. One aspect of successful spatial compounding of the ultrasound images from different angles is to accurately register the ultrasound image frames. Errors in image registration during compounding can cause blurring and possibly loss of the details in the original images. Spatial compounding of image frames from different view angles helps reduce the speckle noises, thus increasing the capability of tissue differentiation, and also improves the visualization of the boundaries and internal lesions posterior to the boundaries.
U.S. Pat. No. 5,653,235 (Teo, issued Aug. 5, 1997) describes a method of performing spatial compounding of image frames by a two-dimensional array transducer. In that patent, an ultrasound beam is electronically steered to focus on the same region, but from two different directions. The received image frames are combined to form a compounded image. This approach generally lacks flexibility and the number of view directions is limited.
Several methods have been used successfully in ultrasound image registration, including the Sum of Absolute Differences (SAD) technique, the correlation technique and the landmark matching technique.
There is a need for an ultrasound spatial compounding technique in which viewing direction is conveniently determined by an operator and the number of view directions is not limited.
The preferred embodiment is useful for spatial compounding of ultrasound image frames from different view angles. In such an environment, the preferred embodiment comprises transmitting first ultrasound waves toward a subject from a first angle and receiving first reflected ultrasound waves from the subject in response to the first transmitted ultrasound waves. Second ultrasound waves also are transmitted toward a subject from a second angle, and second reflected ultrasound waves are received from the subject in response to the second transmitted ultrasound waves. The transmitting and receiving preferably are accomplished with an ultrasound transducer array or probe. A first frame of data representing a first image is generated in response to the first reflected ultrasound waves, and a second frame of data representing a second image is generated in response to the second reflected ultrasound waves. The generating preferably is accomplished with a processor. At least the first frame and second frame are stored and an image registration algorithm also is stored. The storing preferably is done in a memory. The algorithm is executed to register the first frame with the second frame and to generate a third frame of data representing a registered image. The execution of the algorithm preferably is done with the processor. The third frame of data is displayed, preferably on a display monitor.
By using the foregoing techniques, the viewing direction of the ultrasound waves is flexible and at the discretion of an operator. Moreover, the number of view directions is unlimited. In addition, these techniques are not limited by the type of transducer used.